FIGS. 17(a) and 17(b) are perspective views for explaining a conventional method for measuring the thickness of a semiconductor layer epitaxially grown on a substrate. In these figures, reference numeral 101 designates a GaAs substrate and numeral 102 designates an epitaxial layer grown on the GaAs substrate 101.
Initially, as illustrated in FIG. 17(a), an epitaxial layer 102 is grown on the GaAs substrate 101 having a diameter of 2 inches by MOCVD. The thickness of the epitaxial layer 102 is controlled by the growth time according to the growth rate which is controlled by the quantity of the supplied source gas. Thereafter, this wafer is cleaved to make a strip sample 103 shown in FIG. 17(b), and the section of the sample 103 is observed with a scanning electron microscopy (SEM) and photographed, whereby the thickness of the epitaxial layer is determined.
In the conventional evaluation method, however, the cleaving of the wafer takes much time and labor. In addition, since the evaluation is performed after the growth of the epitaxial layer, a feedback control cannot be applied during the epitaxial growth, so that the controllability of the thickness is poor, resulting in a poor production yield.
FIG. 18 is a schematic diagram illustrating a crystal growth monitor apparatus for optically measuring thickness of an epitaxial layer grown on a semiconductor substrate by molecular beam epitaxy (MBE). This apparatus is disclosed in Japanese Published Patent Application No. Hei. 2-252694.
In FIG. 18, an ultra-high vacuum container 111 having an observation window 112 contains a substrate holder 113 on which a substrate 114 is disposed. In the ultra-high vacuum container a, molecular or atomic beam of materials, which is produced by evaporating material sources 115 and 116, reaches the substrate 114, whereby a crystal is grown on the substrate. Reference numeral 117 designates a light source, numeral 119 designates a half mirror for separating reflected light from the substrate 114, numeral 120 designates a condenser lens, numeral 121 designates a diaphragm, and numeral 123 designates an eyepiece. Light 118 emitted from the light source 117 travels through the half mirror 119, the condenser lens 120, the diaphragm 121, and the observation window 112 and reaches the surface of the crystal layer growing on the substrate. The light is reflected at the surface of the growing crystal layer and received by the half mirror 122. The half mirror 122 is observed with the eyepiece 123 to see if the light strikes the proper position on the surface of the substrate. The optical path is adjusted if necessary. On the other hand, the light reflected at the surface of the growing crystal layer is reflected by the half mirror 119. The reflected light 124 is received by a spectroscope 125 and turned into monochromatic light. This monochromatic light is guided through a condenser lens 126 to a Rochon prism 127. In the Rochon prism 127, the monochromatic light is divided into two beams having different polarization planes that are at right angles to each other. These two light beams are respectively received by PIN photodiodes 128 and 129 having similar characteristics. These photodiodes 128 and 129 are connected in series and in reverse polarity, and the difference output is amplified by a DC amplifier 130 and recorded in a recorder 131.
Polarized light caused by reflection at the surface of the growing crystal layer varies according to the crystal growth condition. For example, in GaAs growth, polarized light attains a maximum when an atomic plane of Ga is formed. Thereafter, the polarized light gradually decreases as As molecules are accumulated on the atomic plane and, finally, becomes a minimum. When an As atomic plane is again formed, the polarized light becomes the maximum again. Therefore, if orthogonal polarization components are taken out and a difference between them is measured, the growth process at the surface of the substrate is directly observed.
FIG. 19 illustrates the result of observation of a growing GaAs crystal using the crystal growth monitor apparatus shown in FIG. 18, in which the abscissa shows the growth time and the ordinate shows the output from the DC amplifier 130. This output corresponds to a difference in intensities of the orthogonal polarization components. In addition, the output from the DC amplifier 130 shows a damped oscillation, and a period of this damped oscillation corresponds to the growth of one atomic layer. Therefore, the thickness of the growing crystal layer can be determined by counting the periods.
In the prior art crystal growth monitor apparatus, however, since the thickness of the growing crystal layer is measured by detecting the variation in the reflected light due to the atomic layer level unevenness at the surface of the crystal layer, the variation in the output signal is very small, resulting in difficulty in the measurement. In addition, in order to increase the S/N ratio, means for polarizing the incident light is required, whereby the monitor apparatus is complicated and rises in price.
FIGS. 20(a) and 20(b) are perspective views for explaining a prior art method for evaluating an AlGaAs multiple quantum well (hereinafter referred to as MQW) layer. In these figures, reference numeral 201 designates a GaAs substrate. A first Al.sub.0.4 Ga.sub.0.6 As layer 202 is disposed on the GaAs substrate 201. An MQW layer 203 comprising alternating Al.sub.0.1 Ga.sub.0.9 As well layers and Al.sub.0.3 Ga.sub.0.7 As barrier layers is disposed on the first Al.sub.0.4 Ga.sub.0.6 As layer 202. A second Al.sub.0.4 Ga.sub.0.6 As layer 204 is disposed on the MQW layer 203.
A description is given of the evaluation process.
Initially, as illustrated in FIG. 20(a), there are successively grown on the 2-inch diameter GaAs substrate 201 the first Al.sub.0.4 Ga.sub.0.6 As layer 202, the MQW layer 203 comprising alternating Al.sub.0.1 Ga.sub.0.9 As well layers and Al.sub.0.3 Ga.sub.0.7 As barrier layers, and the second Al.sub.0.4 Ga.sub.0.6 As layer 204, preferably by MOCVD. The thicknesses of these layers are controlled by the growth time according to the growth rate which is controlled by the quantity of the supplied source gas. Thereafter, as illustrated in FIG. 20(b), the wafer is cleaved to make a strip sample 205, and the section of the sample 205 is observed with an SEM and photographed, whereby the thicknesses of the respective layers are evaluated. Further, the Al compositions of the respective layers are evaluated by peak wavelengths obtained in a PL (photoluminescence) evaluation at room temperature.
In the above-described evaluation process of the thickness of the AlGaAs MQW layer, however, the cleaving of the wafer takes much time and labor. In addition, sufficient precision to detect an error of several nanometer cannot be achieved. In evaluation of Al composition, the PL peak wavelength varies due to variations in the thickness and the Al composition, and the variation in the Al composition cannot be separated from the variation in the thickness. Therefore, an accurate evaluation of the Al composition is impossible.